In recent years sublimation transfer printing, especially aqueous inkjet sublimation transfer printing, has grown substantially because of its cost-effectiveness, its ability to easily allow mass customization of substrates and its eco-friendly nature. The fast development of inkjet technology has contributed to the growth of sublimation inkjet printing. Sublimation transfer printing is widely used for synthetic polymeric materials such as polyester, polyurethane and the like because of the affinity between the sublimation colorant and the polymer. This feature enables the broad imaging application in synthetic textile/fabric imaging, and decoration of various surfaces comprising synthetic polymer materials.
Transfer printing processes involve physically transferring a printed image from one substrate to another, which is accomplished in some embodiments by the application of heat and pressure to one or both substrates. In the case of sublimation heat transfer printing, sublimation colorant molecules are embedded in the substrate material, often a polymer matrix, to form color or monochromatic images. The process is accomplished by heating colorants to gasification temperature, usually under pressure, and transferring the gasified colorant molecules from a transfer media to a final substrate, wherein the molecules either sublimate or migrate into the polymer matrix through the surface of the polymeric material for which the colorants have an affinity.
The very nature of the process of creating sublimation printing images means that at least some of the colorant molecules are present on the surface of the polymer material of the substrate, where they are exposed to a harsh environment, leading to discoloration. Chemicals such as oxygen, peroxide, and solvents/grease, and mechanical forces, heat and/or other energy sources, such as radiation, etc., are among the common factors that may be detrimental to preservation of the image/colorant at the surface of the substrate. If there are no other thermally migratable ingredients in the matrix, the sublimation colorants will continue to migrate according to the concentration gradient until they are exhausted or substantially exhausted.
Attempts have been made to protect the polymer coating and to extend the life of polymer coatings from weathering where both thermal and radiation damage contribute to colorant and image depreciation. Chemicals are used as additives to enhance weather-resistance. However, these chemicals are designed and are used to primarily protect the coating material, i.e. polymer coating materials, and/or the structure underneath the polymer coating material. Mechanical strength and integrity are of foremost importance for outdoor applications, such as construction materials, equipment, furniture and/or appliances, to reduce of elimination harmful effects from exposure to mechanical, radiation or chemical depreciation.
There is a need for improved protection of images formed by sublimation colorants and residing within a substrate polymer matrix from the effects of thermal and radiation damage. Even though coatings with various additives have been used for sublimation printing, previously known and used additives are not tailored to specifically protect sublimation colorants due to the lack of understanding sublimation imaging processes, and the physical or chemical properties of these colorants inside the coating and the surface of the coating. For example, conventional coating additives decompose at high temperatures during heat transfer of sublimation colorants, which may occur at temperatures exceeding 200° C. Chemicals used in coatings may harm the quality of the color image or the longevity of image. Colored additives or non-transparent radiation blocking agents may substantially and negatively impact image quality, largely due to the transparent/translucent nature of sublimation colorants. Monomers, oligomers, solvent residues, or chemicals with catalytic properties may result in accelerated oxidation or reduction reaction of sublimation colorant molecule chromophores. Further, additives that are deeply embedded in the coating layers do not contribute to protection of these colorants, especially protection from radiation.
Digital imaging, such as inkjet printing, has intrinsic benefits. In addition to economic benefits, digital printing provides a larger selection of colors, increased color space, higher image acuity and precision and finer detail. On-demand ink droplets are delivered in very minute and accurate discharges in required areas. Inkjet printing is especially useful for delivery of liquid phase chemicals or ingredients in required sequences to create either physical coverage/insertion, or chemical reactions on a micro-scale, which eventually leads to enhanced quality of the finished product. However, these techniques have not been fully utilized for enhancing sublimation image quality, especially in the area of radiation resistance.